"Arrow," the founding series of The CW's interconnected DCTV universe, is about to reach its 100th episode. It's a significant achievement and one that has given the show's executive producer Marc Guggenheim an opportunity to reflect on the various challenges the series has faced in its first five years.

Speaking with TVLine.com, Guggenheim revealed that one difficulty the show faced early on was costume design -- specifically the design for the iconic mercenary Deathstroke.

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“Death stroke was probably the hardest, believe it or not," the "Arrow" show runner said. "In fact, we actually reshot pieces of the first episode that he appears in, because of the helmet. I don't think [costume designer] Maya Mani and [property master] Ken Hawryliw, who did the helmet, get enough credit for how difficult that look is to achieve in three dimensions."

"Actors are annoying — they have noses and they need to breathe, and they need to see, and all these things are very, very difficult to achieve in that design," Guggenheim continued, "but they just nailed it. That was definitely one of the hardest [costumes], but also one of my favourites."

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Starring Stephen Amell as the Emerald Archer, “Arrow” airs Wednesdays at 8 pm ET/PT on The CW. The series also stars Emily Bett Rickards, David Ramsey, John Barrowman, and Willa Holland.